This invention relates to hybrid circuits and more particularly to a method of forming an improved connector assembly for use with thick film electrical circuits.
Beginning with the invention of the point-contact transistor in the late 1940's and spurred by the development of the first integrated circuits in the late 1950's, there has been a continuous trend in the electronics industry toward miniaturization of electronic assemblies. Totally integrated monolithic circuits now enable the fabrication, for example, of an entire computer on a single slice of silicon. Although found in widespread use, monolithic circuits impose various design constraints which must be observed. Among other factors, they are extremely expensive to fabricate in small quantities, they do not perform well as high frequencies, e.g. in the microwave region, and they are essentially low power devices.
The hybrid integrated circuit overcomes some of the foregoing design constraints while maintaining the objective of circuit miniaturization. Hybrid integrated circuits are normally classified into two basic types--the thick film hybrid circuit and the thin film hybrid circuit. In thick film circuit fabrication, screen printing techniques are conventionally used to deposit selected geometries of conductive, resistive, and capacitive pastes on a ceramic substrate. The deposited patterns are subsequently fired at a high temperature, generally between 700.degree. and 1000.degree. C, to form a hard film approximately 1 mil thick. Discrete capacitors and active circuit elements such as transistors may also be included on the substrate of the circuit. On the other hand, thin film circuits are normally formed by vaporizing thin film materials in a high vacuum which are deposited as a thin coating onto a glass or a ceramic substrate, usually in thicknesses of 0.10 mils or less. Patterns are then selectively etched into the conductive and resistive coatings to form resistors with connecting conductive paths.
In the case of both thick and thin film hybrid circuits, it is typically necessary to provide means for connecting and/or disconnecting the circuit from the remainder of the electrical system with which it is intended to operate. For this purpose, a series of conductive terminal pads, often formed at the same time and continuously with the conductive film on the substrate, are typically disposed along one edge of the substrate to provide access to the circuit. A series of relatively rigid elongated conductive connectors are then connected, in both a mechanical and electrical sense, to the terminal pads for providing a facility whereby the circuit may be conveniently plugged into or out of, for example, a suitably configured printed circuit board or, alternatively, an appropriate interfacing socket.
It has been generally accepted by prior art workers in this field that merely soldering a connector to the terminal pad will not produce a connection exhibiting sufficient mechanical strength to result in a commercially acceptable product. In particular, it has been found that connectors merely secured to the substrate terminal pads by solder connections tend to peel from their solder fillets much too easily when handled even in a careful manner. Accordingly, where soldering techniques are used to join a connector to a substrate terminal pad, it is conventional practice to protect the connection by encapsulating the entire circuit in an organic plastic package or by utilizing one of various other well known mechanical aids in an attempt to strengthen the connection. For example, connectors having clips at one end adapted to engage the substrate and mechanically strengthen the connection between a connector and its associated terminal pad are well known in the art. Other prior art approaches using ancillary mechanical aids to strengthen the connector-terminal pad connection include that of providing a flanged connector insertable through a suitable aperture formed in the terminal pad. The connector and terminal pad aperture are configured for creating an effect whereby the connector flange cooperates with the substrate to mechanically strengthen the connection. While only two mechanical strengthening aids have been specifically described above, it will be appreciated that a variety of other similar techniques are in common use.
Particularly on a mass or volume production basis, the costs associated with the provision and installation of the foregoing interfacing connectors forms a significant portion of the entire cost of producing a completed hybrid circuit. In thick film circuits, for example, the connectors themselves contribute the highest material costs associated with the entire assembly. Moreover, to a large extent the high material costs associated with the connectors result from the inclusion of the heretofore considered necessary ancillary connection strengthening aids discussed above. These aids have been referred to herein as being ancillary since, for purposes of achieving a good electrical connection, a solder connection is nevertheless generally formed between the connector and the terminal pad. In order to minimize these costs, and thereby the cost of the completed hybrid assembly, the present invention provides a method for firmly securing a plurality of relatively rigid conductive connectors to the terminal pads of a hybrid circuit substrate without the use of ancillary connection strenghtening devices.